1. Field of the Invention
The present invention relates to a route guidance server, navigation apparatus, route guidance system, and route guidance method that are suitable for route guidance of a vehicle.
2. Description of the Related Art
In recent years, navigation apparatuses (hereinafter referred to as navigation apparatuses) are significantly spread, wherein the car navigation apparatuses receive latest information, such as the crowded state of a road, transmitted from various information centers via broadcast or communication devices, and provide the driver of a vehicle with an appropriate route guidance to a destination, taking into account the received latest information.
For example, functions to receive latest traffic information, such as information on a traffic congestion or travel time, provided from a traffic information center via FM multiplex broadcast or the like, to calculate a route with a minimum time to a destination, taking the traffic information into account, and to display the route to the destination on a display, can be said to have been established as basic functions of a navigation apparatus. Today, such a navigation apparatus has been developed that obtains predicted traffic information on the estimated passing time of the vehicle so that the arrival time can be predicted with higher accuracy for a route that allows earlier arrival at the destination. Then, the navigation apparatus uses the obtained predicted traffic information to calculate an optimum route to the destination. Further, navigation apparatuses have been also commercialized that obtain an optimum route calculated by an information center, reflecting latest traffic information, and provide a route guide to a destination.
Further, not limited to traffic information, there is a movement for improvement of navigation service, taking huge and latest information, which external various sites (information centers) have, into a terminal. For example, portal sites that support travel planning at home provide information including route information to a destination, departure time, and POI like resting spots, wherein latest information, such as facility and road usage charges, facility usage status, event information, which dynamically change, are taken into account. There are also services which transfer travel information planned by such a portal site to a navigation apparatus through communication or mobile storage medium, and make the information usable for guided guidance of a route to the destination on the navigation apparatus.
Incidentally, as methods of expressing route information to a destination, there are two methods as follows.    (a) A method that uses the link numbers of road links or node numbers of nodes (which are ordinarily intersections) being connection points of the road links used in map data, and thereby expresses a route as a series of the plural link numbers or node numbers included in the route.    (b) A method that expresses a route by a sequence of points and expresses the route as a series of position coordinates such as the longitudes and latitudes of the respective points.
Method (a) is desirable for correctly reconstructing route information, given by an information center, on a navigation apparatus. However, if the navigation apparatus is produced by a different manufacturer, the format of map data is different, or the map version is different even with the same map data format, it is not always that the same road link numbers are given for the same road. Consequently, when the information center provides route information to the navigation apparatus, Method (a) will cause a problem of lacking in compatibility of road link numbers therebetween.
In contrast, by Method (b), the route information is expressed by coordinate information on latitudes and longitudes of the respective points of a sequence of points representing a route, and accordingly the problem of compatibility of the format of map data and road link numbers is not caused. However, by this method, map data are different between the information center and the navigation apparatus, and further, in a case, for example, where there are roads going close and parallel to the route or roads branching from the route, coordinate positions designated by the information center sometimes cannot be correctly related to road positions of the map data on the navigation apparatus due to the errors between the coordinate information on the longitudes and latitudes of road positions expressed in the map data of the information center and the coordinate information on the latitudes and longitudes of road positions expressed in the map data of the navigation apparatus. That is, Method (b) has a problem that route information transmitted by the information center cannot always be correctly reconstructed by the navigation apparatus.
Further, in the case of Method (b), because route information is expressed by the data of a sequence of many points expressed by coordinate information on latitudes and longitudes, a problem will be caused that the information amount becomes large. When the information amount of route information transmitted from the information center to the navigation apparatus becomes large, the communication load of the communication path will be increased for it as well as the communication cost.
In this situation, as a method of reducing an information amount in such communication, the coordinate information on shape points of a route is often reduced or a road shape is often approximated by a line or curve, and Patent Document 1, namely, Japanese Patent Laid-Open No. 2002-228468 discloses an example of such a case.
According to Patent Document 1, an information center transmits node information on nodes (coordinate information+attribute information, etc. of nodes) which are intermittently selected from node information on nodes included in route information to be transmitted. Based on the node information (coordinate information) on the nodes intermittently selected, a navigation apparatus determines the node numbers, on the map of the navigation apparatus, of the nodes selected intermittently, by map-matching. Then, the navigation apparatus seeks the node numbers of the nodes whose node numbers have been determined, in other words, the node numbers of nodes included in a route connecting the nodes selected above, in other words, the node numbers of the nodes in a fewer number, by route search. Thus, the navigation apparatus reconstructs the route designated by the information center.
Further, according to Patent Document 1, it is judged that mere map-matching of node position coordinates cannot avoid occurrence of errors, and matching of attribute information is also performed such that, not only coordinate information on nodes, but also attribute information (shape information and the like) on the nodes and attribute information on the incoming links/outgoing links at the nodes, are included in node information transmitted from the information center.